Semiconductor image sensing devices are widely used for capturing images in a variety of applications such as digital cameras, camcorders, printers, scanners, etc. Such devices include image sensors that capture optical information and convert the optical information into electrical signals, which are then processed, stored, and otherwise manipulated to result in projection of the captured images onto a display or print medium.
There are generally two categories of image sensor devices that are widely used: charge coupled devices (CCD) and CMOS image sensor (CIS) devices. CCD image sensors operate with low noise and with high uniformity, but generally require higher power consumption and lower speed operation than CIS devices. The features of lower power consumption and higher speed capability are important factors when the image sensors are employed in portable electronic devices such as wireless telephones that include an integrated digital camera. In such applications, CIS devices have become the preferred choice over CCD devices.
A CIS device includes an active pixel sensor (APS) array including a two-dimensional array of photoelectric conversion elements, a timing generator that generates a timing signal for reading signals from the APS, a row driver for selecting pixels for reading, a corrected double sampling (CDS) unit for performing a corrected double sampling procedure on the output signals of selected pixels, a comparator for comparing the CDS-corrected signal with a reference signal, an analog-to-digital converter (ADC) for converting analog signals output by the comparator to digital signals, a digital signal processor (DSP) for converting the converted digital signals to digital image signals and an interface unit which receives command signals from the output device and which outputs the digital image signals to the output device. In different configurations, the DSP processor can be integral with the CIS unit or may be provided on a device that is physically separated from the CIS unit.
In contemporary, high-quality CIS devices, each unit pixel of the APS includes a photoelectric conversion element, such as a photodiode or photogate, for collecting the energy of the incident signal, and either three or four readout transistors, depending on the technology type. In a CIS device with four transistors, the readout transistors include a transfer transistor, a select transistor, a drive transistor and a reset transistor. The readout transistors operate to manage and transport the energy received at the photoelectric conversion element to provide corresponding data to the processing devices for image processing. An array of microlenses are formed over the APS pixel array, each microlens corresponding to a pixel of the array, for concentrating the incident energy on the corresponding photoelectric conversion element.
With the continued trend toward further semiconductor device integration, and with the desired increase in resolution of imaging devices, device “fill factor” is of increased importance. The “fill factor” of an imaging device is the proportion of the area of the photoelectric conversion element of a pixel of the device to the total device area designated to the pixel. Greater fill factor is desired, since this equates to more of the available active region of a device being devoted to actual photoelectric conversion of optical signals, and less of the available active region being designated to readout of the signals. Since a CIS device requires three or four readout elements per unit pixel, the CIS device has a lower fill factor than a CCD device. As the resolution of a CIS device is increased, for example from 1 megapixels to 5 megapixels per unit area, the area of a unit pixel in the CIS device should be reduced. However, further reduction of the area required for the three or four readout elements required for each pixel is limited, due to limitations in the minimization of the sizes of the transistor components of the readout elements, since noise increases with smaller element size. Therefore, as CIS device resolution is increased, the device fill factor is generally reduced.
To alleviate the relatively low fill factor in high-resolution CIS devices, a shared-type CIS sensor has been developed. In such shared devices, neighboring photoelectric conversion devices are configured to share one or more of the readout elements. Such a shared configuration is effective for improving the device fill factor; however, this shared configuration also introduces the problem of misalignment between microlenses formed on the devices, and corresponding photoelectric conversion elements. This is because the conventional shared-type CIS sensor can have a different pitch between neighboring photoelectric conversion elements because of the shared readout elements, either in the row direction, in the column direction, or in both the row and column directions. At the same time, because of the manner in which it is fabricated, the microlens array is generally configured to have a constant pitch in both the row and column directions. Thus, misalignment occurs between the microlens array and the pixel array, degrading image quality.